Ketene-forming eliminations from ArCH
2CO
2C
6H
3-2-X-4-NO
2 (Ar = thienyl,
1) promoted by R
2NH/R
2NH
2+ in 70 mol % MeCN(aq) have been studied kinetically. When X = CF
3 and NO
2, the reactionsexhibit second-order kinetics as well as
![](/images/gifchars/beta2.gif)
= 0.30-0.64 and
![](/images/entities/verbar.gif)
lg![](/images/entities/verbar.gif)
= 0.31-0.52 that decrease with a betterleaving group. Hence, an E2 mechanism is evident. As the leaving group is made poorer (X = H, OCH
3,and Cl), E2 transition state becomes more skewed toward the proton transfer, as revealed by the increasein Br&
ouml;nsted
![](/images/gifchars/beta2.gif)
to 0.5-0.64, and the E1cb mechanism competes. The changes in the
k1 and
k-1/
k2 valueswith the reactant structure variation provide additional support for the competing E1cb mechanism. Bycomparing with existing data for 4-YC
6H
4CH
2CO
2C
6H
3-2-X-4-NO
2, the effect of
![](/images/gifchars/beta2.gif)
-aryl group on ketene-forming elimination is assessed.